Difference between SIFS, PIFS, DIFS, EIFS and AIFS
This page describes difference between SIFS, PIFS, DIFS, EIFS and AIFS used in WLAN standard as per IEEE 802.11.
As we know interframe spacing is very important to coordinate access to the common medium of transmission. This medium carry 802.11 compliant ethernet packets. IEEE 802.11 WLAN standard defines various interframe frames such as SIFS, PIFS, DIFS and EIFS.
The figure mentions three of these four types used for medium access. The fundamental concept in 802.11 MAC to avoid collision is to delay the transmission until medium becomes idle.
Different types of traffic requires different priority levels. This can be achieved by varying spacing between the frames. The idea here is to provide less delay for high priority traffic. Hence high priority traffic can use network before low priority one as soon as it is available.
To help in interoperability between different data rates and vendor products interframe space is fixed for some duration. Let us understand basics of SIFS, PIFS, DIFS and EIFS.
The interframe spacing timings are defined as time gaps on the medium. The interframe spacing timings except AIFS are fixed for each PHY layer.
SIFS-Short Interframe Space
This type is used for RTS/CTS and for positive ack based high priority transmission. One SIFS duration elapses, the transmission can immediately starts. The medium will be busy after the SIFS period is over and hence this type of transmission will have higher priority over others initiated after longer time than SIFS.
Extended rate PHYSICAL LAYER conforming to Clause 19 such as CCK, DSSS and OFDM will use SIFS of value equal to 10 µs.
Following table lists out SIFS times for various physical layer configurations.
| WLAN Physical layer | SIFS value |
|---|---|
| FHSS | 28 µs |
| DSSS | 10 µs |
| OFDM | 6 µs |
| HR/DSSS | 10 µs |
| ERP | 10 µs |
PIFS-PCF Interframe Space
It is used by PCF during the contention free operations. After the PIFS period gets elapsed stations having data to be transmitted in contention free period can be initiated. This will pre-empt any contention based traffic.
PIFS can be used either by
• stations(STAs) operating under PCF to gain priority access of the medium at the start of
CFP(contention free period).
• Station(STA) to transmit channel switch announcement frame.
PIFS = aSIFSTime + aSlotTime
, where
aSIFSTime and aSlotTime are both constant per physical layer.
The aSIFSTime period and aSlotTime are calculated using following parameters.
aSIFSTime = aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime
aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime + aMACProcessingDelay
DIFS-DCF Interframe Space
DIFS is the minimum medium idle time and is used for contention based services/applications. WLAN compliant Stations can have access to the medium immediately if it is free for the time period longer than the value defined as DIFS.
DIFS = aSIFSTime + 2*aSlotTime
EIFS-Extended Interframe Space
This EIFS type is not having fixed interval value. It is used only when there is erroneous frame transmission.
EIFS = aSIFSTime + DIFS + ACKTxTime,
Where,
ACKTxTime is the time expressed in microseconds required to transmit an ACK frame.
ACK frame includes preamble, PLCP header and any additional PHY dependent information,
at the lowest PHY mandatory rate
AIFS-arbitration interframe space
The AIFS shall be used by QoS STAs to transmit all data frames, management frames e.g. MMPDUs and control frames e.g. PS-Poll, RTS, CTS.
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